1. Field of the Invention
The present invention relates generally to systems and methods for receiving spread spectrum radio signals, such for example as digitally modulated signals in a Code Division Multiple Access (CDMA) communication system, propagating through a multipath environment and, more particularly, to a rake receiver scheme for such signals.
2. Description of the Related Art
Direct Sequence Spread Spectrum (DSSS) systems, such as Direct Sequence Code Division Multiple Access (DS-CDMA) systems, are attracting widespread attention in the personal communication fields such, for example, as digital cellular radio. In a DS-CDMA communication system, both the time and frequency domains may be shared by all users simultaneously. As such, a base station may simultaneously transmit distinct information signals to separate users using a single band of frequencies. Individual information signals simultaneously transmitted may be isolated by each receiving user because of the base station's utilization of unique signature sequences in the transmission of the information signals. Prior to transmission, the base station multiplies each information signal by a signature sequence signal assigned to the user intended to receive the signal. A signature sequence consists of a number of chips. By multiplying an information signal with a signature sequence, the rate of transmissions through the channel increases from the bit rate to the chip rate. To recover a transmitted signal from among those signals transmitted simultaneously in a frequency band, a receiving mobile user multiplies a received signal (containing all transmitted signals) by its own unique signature sequence signal and integrates the result. In so doing, the user identifies that signal intended for it, as distinct from other signals intended for other users.
In wireless communication systems (such as DS-CDMA systems), an information signal is communicated from a transmitter to a receiver via a channel comprising several independent paths. These paths are referred to as multipaths. Each multipath represents a distinct route that an information signal may take in traveling between transmitter and receiver. An information signal communicated via such routes or multipaths appears at a receiver as a plurality of multipath signals, one for each multipath. When multipath time dispersion is present in a CDMA system, the receiver receives a composite signal of multiple versions of the transmitted symbol that have propagated along different paths (referred to as "rays"), some of which may have relative time delays of less than one chip. Because of complex addition of multipath signals, the strength of received signals may vary between very small and moderately large values. The phenomenon of received signal strength variation due to complex addition of multipath signals is known as fading.
Among the techniques used to mitigate the effects of fading in DS-CDMA communication systems is the path diversity technique. Path diversity in DS-CDMA systems entails estimation of the delay introduced by each of one or more multipaths (in comparison with some reference, such as line-of-sight delay), and then using this delay in a receiver structure to separate (or resolve) the received multipath signals. A receiver structure often employed to provide path diversity is the so-called rake receiver, which is well known in the art. See. e.g., R. Price and P. E. Green, Jr., A Communication Technique for Multipath Channels, 46 Proc. Inst. Rad. Eng. 555-70 (March 1958).
In a rake receiver employing a conventional correlation-based channel estimator, correlation values of the signature sequence with the received signals at different time delays are passed through a delay line that is tapped at the expected time delay (dt), i.e., the expected time between receiving echoes. The outputs at the rake taps are then combined with appropriate weights to form the rake receiver output. Such a receiver searches for the earliest ray by placing a tap at T.sub.O, and for a ray delayed by dt by placing a tap at T.sub.O +dt, and so forth. The number of taps depends upon the channel delay spread and is less than or equal to the number obtained by dividing the channel delay spread by the chip period. The rake tap outputs having significant energy are appropriately weighted and combined to maximize the received signal-to-noise-and-interference ratio. The total time delay of the delay line determines the amount of arrival time delay that can be searched. In addition, the accuracy of the channel estimator can only be improved by increasing the bandwidth of the transmitted signal.
Thus, while the path diversity afforded by conventional rake receivers is beneficial in many instances, it may not provide a significant diversity benefit where the range of multipath delay values in these environments is small compared with the duration of a DS-CDMA chip interval (which may be, for example, 1 .mu.s). Because of this, knowledge of delay values obtained by a conventional correlation-based channel estimator alone is insufficient to allow resolution of multipath signals.